a problem about symbolic function diff

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SANKU
SANKU 2014-6-16
编辑： Lucas Carvalho 2017-5-13
syms x(t) y(t); f=x*y; I want get df/dx, how to use command diff? Thank you!

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回答（2 个）

Star Strider
Star Strider 2014-6-16
You do not get df/dx once you have defined x as x(t). You get the Chain Rule:
dfdx = diff(f)
produces:
dfdx(t) =
D(x)(t)*y(t) + x(t)*D(y)(t)
The command diff(f,x) or diff(f, x(t)) throws an error.
Lucas Carvalho
Lucas Carvalho 2017-5-13
编辑：Lucas Carvalho 2017-5-13
In your case, the independent variable is only t, while x and y are dependent of t. To derive the f function partially in terms of x or y, you will need the fulldiff function:
https://www.mathworks.com/matlabcentral/fileexchange/7174-fulldiff-m

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